The invention provides improved braking and railway freight cars by providing the ability to operate at higher than standard levels for full service and emergency braking on loaded cars. Higher brake cylinder pressures can provide higher effective net braking ratios on loaded car and increase the maximum available brake retarding force on such loaded cars. This is of particular importance on steep downgrades, for the increase in available braking force and stopping power can enhance operational performance. Importantly, it is possible to provide this increase in braking force on loaded cars without danger of sliding wheels or overheating wheels. The invention also recognizes and takes advantage of the simultaneous brake response of an electro-pneumatic brake system, in that higher retardation of loaded cars can be obtained without creating excessive run-in forces in the train. Conversely, the slower, serial brake signal speed with pneumatic brakes limits the acceptable loaded car retardation without causing excessive run-in forces among cars in the train.
Almost all interchange freight cars are equipped with standard, two-compartment reservoirs consisting of a 2,500 cubic inch auxiliary reservoir and an approximately 3,500 inch emergency reservoir. In normal operation these reservoirs can be charged with compressed air from the brake pipe to approximately the same operating pressure as the brake pipe, typically between 70 and 110 psi. With conventional freight brake equipment, there are two different modes of braking, service and emergency. Service braking is the normal mode and is typically used to balance grade accelerating force and to control speed or stop the train. The compressed air stored in the auxiliary reservoir is used to supply the brake cylinder pressure called for during service braking operations. When a fill service brake application is made, the auxiliary reservoir pressure flows into the brake cylinder, reducing the auxiliary reservoir and increasing the brake cylinder pressure until the two pressures equalize. This generally represents the maximum service brake cylinder pressure available, i.e., a fall service application.
Emergency brake applications are used generally only when unusual circumstances require the train to be braked at the maximum. Emergency applications transmit through the train more rapidly than service applications, and in compliance with regulations, must provide 15% to 20% higher brake cylinder pressure than full service applications. When an emergency application is made in prior art equipment, the higher brake cylinder pressure is obtained by generally simultaneously connecting both the auxiliary reservoir and the emergency reservoir with the brake cylinder. This causes the pressure in all three chambers to equalize at a controlled rate. Normally no higher brake cylinder pressure can be obtained than this emergency equalization pressure, which is typically 17% to 19% higher than a full service equalization pressure. The difference is determined by the relative volumes of the reservoirs.
Theoretical full service and emergency equalization brake cylinder pressures for conventional freight brake equipment are listed on Chart 1. Some minor variations are common in actual brake cylinder pressure, as a result of allowable variations in piping volumes and the brake cylinder piston travel. Chart 1 shows typical ratios between emergency and fill service brake application pressures based upon initial brake pipe and system pressure.
CHART 1 ______________________________________ Initial System Service Emergency Ratio Pressure Equalization Equalization Emer./Service ______________________________________ 70 psi 51.0 psi 60.8 psi 1.192 80 psi 58.7 psi 69.7 psi 1.187 90 psi 66.4 psi 78.6 psi 1.184 100 psi 74.1 psi 87.5 psi 1.181 110 psi 81.7 psi 96.4 psi 1.179 ______________________________________
The relative braking forces used on freight cars is reflected in the net braking ratio of the cars. The net braking ratio is the total actual force developed to apply the brake shoes to the treads of the wheels divided by the weight of the car. The net braking ratio on a fully loaded car with 50 psi in the brake cylinder or cylinders represents the design braking ratio of the car. The braking ratio at any given brake cylinder pressure for a particular state of car loading can be considered to be the effective braking ratio for those particular conditions. Typically, the total net shoe forces are not proportioned to car weight through the full possible load range of the car, and therefore the empty car braking ratios are much higher than the loaded car braking ratios. Typical loaded car braking ratios vary from 6.5% to 10% at 50 psi brake cylinder. Empty car net braking ratios can vary from 20% to 30% for brake cylinder pressures that may be 50 psi or may be reduced from 50 psi by empty/load equipments. For a given coefficient of brake shoe friction, the potential braking deceleration of freight cars is generally proportional to their effective net braking ratios. Potential deceleration is the braking deceleration which would occur if the car was not in a train with other cars and locomotives, that is, its independent deceleration based upon its own onboard brake system without acceleration or deceleration imported from adjacent cars through the respective couplings.
The amount of brake cylinder pressure and resultant shoe force that can be safely and effectively used on railway freight cars is limited by a number of factors. The ultimate limit is the pressure and shoe force which produces sufficient retarding force to exceed the available rolling adhesion and risk a locked up condition in which the steel wheels slide on the steel rails. Another limiting factor with friction braking on wheel treads is the thermal capacity of the wheels to absorb and dissipate the heat created by braking without damage to the wheel. A third and significant limiting factor is the level of longitudinal buff and draft forces that can be created among cars in a train during braking. Finally, there are practical limitations on pneumatic pressures, physical reservoirs and cylinder sizes and mechanical leverage ratios that can be employed.
As a result of these and other limitations, braking forces typically employed on freight cars are not high enough to create a risk of sliding wheels unless the car is empty or very lightly loaded. With loaded trains there are two primary limitations of the maximum braking forces or braking ratios that can be employed. One is the brake horsepower that can be used on long, steep grades without overheating wheels, and the second is the need to control the longitudinal forces between cars, particularly during emergency braking. The grade braking horsepower can be effectively controlled by limiting the operating speed on the grade, regardless of how much reserve braking is available. Because the grade braking situation usually requires braking control over long periods of time, the wheel heating limitations previously discussed may not permit a higher level of braking and, therefore, grade braking is usually somewhat less than full service.
The primary limitation on braking for loaded cars, therefore, is the need to control longitudinal forces, in-train dynamic forces. The major cause of high longitudinal forces that develop between cars during braking is the serial, front to rear transmission of pneumatic brake signals on long trains. The cars at the front of the train develop effective retarding forces before the brake application signal reaches the rear of the train, which causes the front to begin to decelerate first and the rear cars to progressively drive into and be restrained by the forward cars. Therefore, the higher the effective braking and retardation is on the forward cars in the train, the greater the run-in forces that can be developed within the train using pneumatic braking. Substantially higher brake cylinder pressures and braking forces could be used safely on longer loaded trains only if a faster brake signal transmission was implemented. The use of electro-pneumatic braking can effectively increase the brake command signal transmission and effectively retard the rear cars sooner and prevent severe run-in conditions.